In current cellular mobile broadband systems the achievable data rates are strongly dependent on the users' positions in the network.
Even though it is of great importance to deliver the same user experience across the whole cellular network in order to meet the users' expectations, still, a considerable gap is observed between cell-edge and cell-core performance due to inter-cell interference, which poses the main limitation of state-of-the art mobile networks.
Long Term Evolution (“LTE”) is the 4th generation cellular mobile system that is being developed and specified in 3GPP as a successor of the Universal Mobile Telecommunications System (“UMTS”) standard which was adopted by third generation mobile cellular systems for networks based on the GSM standard. LTE is specified as frequency reuse-1 system designed to achieve maximum gain and efficient use of frequency resources. On one hand, the optimal use of resources provides higher bit rates while on the other hand it generates Inter Cell Interference (ICI) issues associated with the reuse-1 type of deployment. In the absence of any interference mitigation or coordination mechanism, ICI becomes critical in LTE, and as described above, especially on cell borders. Therefore, a number of schemes have been suggested for the mitigation solution of ICI, and are typically classified as static and dynamic on the basis of their type of interference coordination mechanisms.
One of these types is centralized ICIC (“cICIC”) which has the advantage of addressing interference issues that distributed ICIC (“dICIC”), which is implemented at the eNodeB level, is incapable of handling.
The evolution of the physical layer of the cellular radio access has reached nowadays a level where operation close to theoretical limits of achievable spectral efficiency for a given signal to interference-and-noise (SINR) ratio, becomes feasible. Thus, significant increases in spectral efficiency can be achieved only by improving the SINR through minimization of the interference.
The 3GPP LTE Recommendation defines two types of interference minimization techniques. The first one being interference minimization by interference reduction, whereas the second one is interference minimization by inter cell interference coordination (ICIC). The 3GPP standard handles the two types of interference minimization differently. The first type, interference reduction, is used in conjunction with coverage and capacity optimization. The interference reduction is done by implementing RF techniques such as antenna tilt, transmit power reduction, and handover mechanisms. The second type, ICIC, is used exclusively for cell edge user equipment (UE), to which the same Physical Resource Blocks (PRBs) have been assigned by the serving wireless cell as those assigned in other wireless cells to their associated UEs that cause the interference.
The LTE Recommendation has defined a new interface between base stations to enable the transfer of ICIC function indicators. This interface is referred to as X2. These function indicators are: Relative Narrowband Transmit Power Indicator (“RNTPI”), High Interference Indicator (“HII”), and Interference Overload Indicator (“OI”).
The RNTPI indicator message is sent to neighbor base stations (referred to herein as “eNBs”). It contains one bit per each Physical Resource Block (PRB) in the downlink transmission, which indicates if the transmission power associated with that PRB will be greater than a pre-defined threshold. Thus, neighbor eNBs may anticipate which bands would suffer more severe interference and take the appropriate scheduling decisions immediately, rather than wait to receive and rely on the UEs' Channel Quality Information (“CQI”) reports.
The HII indicator for uplink transmissions has a somewhat similar function as that which was described above in connection with the RNTPI message for downlink transmissions. There is one bit per each PRB, enabling the neighboring eNBs to assess whether they should expect high interference power in the near future. Typically, only PRBs that are assigned to cell-edge UEs are indicated by these messages. Reference Signal Received Power (“RSRP”) measurements which are reported as part of handover measurement reports, can identify cell edge UEs. In a similar manner, this indicator can be used to identify the bands used in a frequency partitioning scheme.
While the previously described X2 messages are sent out proactively by the eNBs, the overload indicator (“OI”) is only triggered when high-interference in the uplink direction is detected by an eNB. In such a case, an overload indication will be sent to neighbor eNBs whose UEs are potentially the source of this high interference. The message contains a low, medium or high interference level indication per each PRB. However, the question, which cell is the one responsible for the high interference is of course not a trivial question to answer.
According to 3GPP TS 36300-970, Inter-cell interference coordination is associated with managing radio resources (notably the radio resource blocks) such that inter-cell interference is kept under control. ICIC is inherently a multi-cell, radio resource management (“RRM”) function that needs to take into account information (e.g. the resource usage status and traffic load situation) obtained from various cells. Furthermore, an ICIC method may be different in the uplink and downlink.
3GPP release 10 introduces a new LTE network concept which is defined as heterogeneous networks (“HetNet”), in contrast to previous network releases which deal with homogeneous networks. HetNet is defined as a network of eNBs with different capabilities, most importantly, different Tx-power classes.
However, heterogeneous networks pose new ICIC challenges. A first ICIC challenge involves Macro UE that roams about a Home eNB (HeNB) and is not part of the closed subscriber group (“CSG”). In that scenario the Macro eNB UE transmission will become uplink interference to the Home eNB authorized UEs. The second ICIC challenge is Macro eNB transmission to cell edge UEs that forms downlink interference to Pico eNB center cell UE. In order to enable the use of HetNet, enhanced ICIC (eICIC) Rel. 10 requires that all members of a HetNet (Macro, Pico, HeNB) should be capable of interconnecting by using the X2 interface.
Another major problem is that the ICIC is limited to data channels. Therefore, the recommendation does not provide sufficient protection for the downlink control channels in the two above-mentioned severe interference scenarios. Furthermore, range expansion has to be limited to small offsets between cells, in order to keep control channel errors at a reasonable level. Hence for Rel. 10 3GPP two new approaches were proposed to avoid heavy inter-cell interference on both data and control channels in the downlink direction. One is based on carrier aggregation with cross-carrier scheduling, while the other is based on time-domain multiplexing (“TDM”) using so called almost blank sub-frames (“ABS”).
Carrier Aggregation is one of the most important features of the LTE Advanced. Unlike LTE, it enables an LTE-A UE to connect to several carriers simultaneously. It not only allows resource allocation across carriers, it also allows implementing a scheduler based on fast switching between carriers without time consuming handover.